Taxonomy & naming
Jenynsia aurea was described in 2026 by Cristine Hartmann, Juliano M. Wingert, Ricardo C. Angrizani and Luiz R. Malabarba, in Neotropical Ichthyology (volume 24, issue 1, article e250128). It belongs to the family Anablepidae — the four-eyed fishes and one-sided livebearers, within the order Cyprinodontiformes — and specifically to the genus Jenynsia, subgenus Plesiojenynsia, one of two subgenera recognised within Jenynsia alongside the nominate Jenynsia. Species of Plesiojenynsia are restricted to clear-water streams over basalt beds in the southern Serra Geral Formation of Brazil, in contrast to the sand-associated, often estuarine species of the nominate subgenus.
The holotype is a 2.23 in SL male (UFRGS 29990) from arroio Carvalho, a tributary of the rio Três Forquilhas in Itati, Rio Grande do Sul, with a large paratype series at UFRGS and MCP. Eschmeyer's Catalog of Fishes treats Jenynsia aurea Hartmann, Wingert, Angrizani & Malabarba, 2026 as a valid, currently accepted name. Before this description, fish from the rio Maquiné and rio Três Forquilhas drainages had been identified as Jenynsia unitaenia, a species originally described from the neighbouring rio Mampituba drainage; one lot of paratypes from the rio Três Forquilhas cited in the original 1995 description of J. unitaenia in fact belongs to what is now recognised as J. aurea.
The describing team used mitochondrial DNA sequencing (cytochrome oxidase subunit I) and morphological comparison across four river systems of the Tramandaí-Mampituba ecoregion — the rio Araranguá, rio Mampituba, rio Três Forquilhas and rio Maquiné — to show that populations split into two genetically and morphologically distinct lineages roughly 1.2 million years ago, during the Pleistocene, likely driven by cycles of marine transgression and sea-level change along the South American coast. The rio Mampituba and rio Araranguá populations retain the name J. unitaenia; the rio Maquiné and rio Três Forquilhas populations, diverging from each other only about 0.2 million years ago, were described as the new species J. aurea. The two species are diagnosed chiefly by colour pattern and by a difference in the shape of the anterior cleft in the anguloarticular bone, since standard morphometric and meristic measurements show broad overlap between them.
Jenynsia aurea is distinguished from J. diphyes, J. eigenmanni and J. eirmostigma by possessing a single midlateral stripe in adults, rather than discontinuous or multiple lateral stripes. From its closest relative, J. unitaenia, it differs (except in juveniles) by lacking dorsolateral or ventrolateral chevron marks associated with the midlateral stripe. It is further distinguished from several congeners by gill-raker count and by the presence of the first mandibular-canal pore.
Morphology
Jenynsia aurea is a moderately sized livebearer with a body that is terete and laterally compressed toward the rear, deepest at the pelvic-fin origin (or further forward in pregnant females). The head is relatively short and pointed, the mouth terminal to slightly subterminal and displaced ventrally when protracted, and the dorsal fin originates at a vertical between the pelvic and anal fins. The largest female examined reached 2.88 in SL, and the largest male 2.67 in SL — females attaining a somewhat larger maximum size, as is typical of livebearers that must carry developing young.
Colour is strongly size- and sex-dependent. Mature males over roughly 2.17 in SL are golden brown on the back and upper body and head, golden yellow on the ventral half of the body and on the pectoral, dorsal, anal and caudal fins, with hyaline pelvic fins and a white ventral surface (the holotype is unusually golden yellow ventrally as well). Females are darker: dark brown on the back and upper body and head, golden yellow ventrally, with all fins hyaline. Specimens under about 1.97 in SL of either sex show a plainer light brown colouration. All individuals carry a continuous lateral stripe running the length of the body — hardly visible in fully coloured males, whose scale row along the stripe reflects light over the underlying dark brown band, but distinct in females and juveniles.
As in all Jenynsia, males possess a tubular gonopodium modified from the anal fin, used for internal fertilisation; in J. aurea this structure has a comparatively short fourth ray, less than two-thirds the length of the third ray, a character used to separate the subgenus Plesiojenynsia from the nominate subgenus Jenynsia.
Habitat
Jenynsia aurea occurs in the rio Maquiné, rio Três Forquilhas, and the rio Tramandaí system generally, in the valleys of the Serra Geral Formation in Rio Grande do Sul, southern Brazil. This is the Tramandaí-Mampituba ecoregion, a coastal Atlantic Forest system recognised for unusually high freshwater fish endemism, and one whose biogeographic history has produced varied patterns of relatedness among the different fish lineages that inhabit it.
Species of the subgenus Plesiojenynsia, to which J. aurea belongs, are characteristically found in clear-water streams flowing over basalt beds — a habitat distinct from the fine sand and often estuarine conditions favoured by the nominate subgenus Jenynsia. No specific water-chemistry measurements (temperature, pH, hardness) were reported in the source material available to us, so no numeric range is asserted here beyond the general character of a clear, basalt-associated southern Brazilian coastal stream.
Feeding
No dedicated dietary study of Jenynsia aurea has been published. As general context for the genus rather than a documented finding for this species: Jenynsia livebearers of shallow coastal-plain and foothill streams are typically opportunistic omnivores, grazing algae, detritus and biofilm from submerged surfaces while also taking small aquatic invertebrates. No species-specific feeding data are available to confirm this pattern in J. aurea.
Mating
Jenynsia aurea reproduces by internal fertilisation, the defining reproductive mode of the family Anablepidae. The male's anal fin is modified into a tubular gonopodium used to transfer sperm to the female; in this species, and throughout the subgenus Plesiojenynsia, the gonopodium's fourth ray is comparatively short. As in other Jenynsia, the genital anatomy of both sexes is thought to be laterally asymmetric — the trait that gives the genus its 'one-sided' reputation, in which a male can typically only inseminate females whose anatomy is oriented to the complementary side. No courtship behaviour specific to J. aurea has been documented in the sources available to us.
Breeding
As a member of the genus Jenynsia, J. aurea is presumed to share the genus's viviparous, matrotrophic reproductive mode, in which the mother nourishes developing embryos internally through placenta-like tissue rather than relying solely on a pre-loaded yolk — a pattern well documented in other Jenynsia species and one of the more advanced forms of livebearing among fishes. No brood size, gestation length, or other quantitative reproductive data specific to J. aurea were reported in the source material available to us.
In the aquarium
Jenynsia aurea is not established in the aquarium hobby, and no husbandry record specific to this newly described species is available to us. By analogy with the clear, basalt-stream habitat documented for the subgenus Plesiojenynsia, an aquarium approximating cool-to-moderate, well-oxygenated, clear running water would be a reasonable starting point, but this is inference from habitat description rather than tested aquarium practice, and no aquarium-specific guidance is asserted here.
Conservation
Jenynsia aurea has not been formally assessed on the IUCN Red List, as is expected for a species described in 2026. The describing authors, however, state in the original description that the species is widely distributed in the rio Maquiné and rio Três Forquilhas drainages and that no specific threat can be assigned to it, and on that basis they judge that it would be classified as Least Concern (LC) under IUCN categories and criteria — a recommendation from the primary literature rather than a completed Red List assessment.
The Tramandaí-Mampituba ecoregion in which J. aurea occurs is recognised for high freshwater fish endemism, and while this particular species appears secure across its known drainages, the ecoregion as a whole depends on the continued integrity of the Serra Geral Formation's basalt-stream systems. No specific anthropogenic threat to J. aurea was identified in the source material available to us.